DSL systems which operate at very high bandwidths tend to do so by employing higher frequency portions of the spectrum which becomes more feasible as the length of each line shortens (as fiber is pushed further towards the edges of the access network—e.g. Fiber to the Cabinet (FttC) and Fiber to the Drop Point (FttDP). Unfortunately, at these much higher frequencies cross talk from neighboring lines operating at the same high frequencies becomes more of a problem.
The latest version of ADSL (e.g. as specified in the ITU standard G992.3) provides for low power modes when a line is not transmitting or receiving data and it is expected that such low power modes will be introduced into the VDSL standard at some point before too long. A particular problem of cross noise when a system has low power modes is that the level of cross talk noise will vary significantly when a neighboring line is in a low power mode, compared to when it is in a high power mode and actively transmitting and/or receiving data.
When a DSL line synchronizes up (i.e. a connection is set up across the line between the two DSL transceivers at each end of the line—this process is also known as resynching and retraining) a measurement of the noise is taken and is used to set certain parameters which are then used throughout the connection until a new connection is established (i.e. until the line re-synchs). The most important parameter which is set is the target signal to noise ratio margin (target SNM), which in turn causes the line to synchronize at a particular rate for the given noise detected at each carrier frequency. Once a connection has synchronized at a particular line rate, it will endeavor to maintain that line rate for the duration of the connection. If the noise experienced by the line becomes too great to sustain the line rate at which the line synchronized, the connection will fail and the DSL transceivers will lose synchronization with each other. At this point the line will automatically resynch—quite possibly at a lower line rate if the noise which caused the resynch is still present while the line is resynching.
As noted above, the use of low power modes can cause the noise environment of a DSL line to change dramatically over time. For example, if a first line runs adjacent to second line (e.g. in a bundle of lines running together towards and indeed to a common network-side DSL transceiver (e.g. in a Digital Subscriber Line Access Module (DSLAM)) and if the first line synchronizes at a time when the second line is operating in a low power mode it may experience a relatively mild or noiseless noise environment (compared to what it would have been if the second line were operating in a high power mode) and may consequently synchronize at a fairly high line rate for a given SNM. If at a subsequent time the second line then “wakes up” and goes into a high power mode, the affect on the noise environment of the first line may be such that it can no longer sustain the line rate at which it originally synched and so cause the first line to drop out.
To address the above prospective problem it has been proposed to use a feature called “virtual noise”. As specified in for example ITU's standards G.992.3 (ADSL 2) and G993.2 (VDSL2) the virtual noise is represented as a set of break points and is either sent from one transceiver to another or read by a transceiver from a local Management Information Base (MIB) at the time of setting up a connection. Generally, the connection comprises two transceivers or modems, referred to hereinafter as a Transceiver Unit-User-side (TU-U) and a Transceiver Unit-Network-side (TU-N), and a twisted copper pair between them.
[Note on terminology: in G992.3 the modems are referred to as an ATU-R (standing for ADSL Transceiver Unit-Remote (i.e. at the user's end)) and an ATU-C (for ADSL Transceiver unit-Central office (at the Central Office or Exchange side), whilst in G993.2 (VDSL 2) they are referred to as a VTU-R and a VTU-O (for VDSL Transceiver Unit-central Office). In the present specification they shall be referred to as a Transceiver Unit-User-side (TU-U) and a Transceiver Unit-Network-side (TU-N) but these correspond to the ATU-R and ATU-C of G992.3 and the VTU-R and VTU-O of G993.2 respectively.]
The range of frequencies at which the TU's may communicate over the connection is generally specified by a standard which is applicable to a certain geographical region as well as being governed by the capabilities of the TU's and the standard to which they are operating (e.g. G992.3 or G993.2, etc.). Within this range there are allocated one or more upstream bands and one or more downstream bands. In each upstream band the TU-U (e.g. ATU-R, VTU-R) will transmit signals and the TU-N (e.g. ATU-C, VTU-O) will receive signals. Correspondingly in each downstream band the TU-N (e.g. ATU-C, VTU-O) will transmit signals and the TU-U will receive signals.
During synchronization a series of test signals are transmitted by both transceivers and there is also a quiet line phase when no signals are transmitted by either TU. During this time each TU measures/estimates the noise detected on its respective receive bands (e.g. the TU-U measures noise on the downstream bands while the TU-N measures noise on the upstream bands) for each tone/sub-carrier frequency in the respective band and then reports an indication of this detected/measured noise estimation to the other TU. Each TU then tries (in a rate adaptive system) to determine an overall data rate (which it is reasonable to suppose is sustainable for the duration of the connection) and a bit allocation for each bin/tone/sub-carrier frequency (which allocation may change to take into account variations in the channel or the noise etc. over the duration of the connection) based at least partially on this information. If a virtual noise scheme is being used, the appropriate virtual noise specified for the respective band in question is additionally taken into account when determining a bit allocation for each bin/tone/sub-carrier frequency.
Although the standards specify details of how the virtual noise should be expressed (e.g. as a set of break points with virtual noise for tones/bins not corresponding to a break point being obtained by linear interpolation between the values of break points on either side of the respective bin/tone), no methodology for setting the virtual noise in the first place is provided in the standards since this is a matter which can be determined on a vendor by vendor basis and does not therefore need to form part of the standards.
US 2010/0254442 describes a method of selecting a virtual noise mask for use in training a pair of cooperating DSL modems. The noise mask is selected from one of a set of predetermined templates each of which has a similar pattern or profile across the range of frequencies (used by the modems) but a differing magnitude. (The pattern or profile itself seems to be based on a Power Spectral Density (PSD) mask specified by a band plan or a standard or something similar specified by a National or Regional authority.) The exact template to select is determined based on a series of measurements of the Signal to Noise Ratio (SNR) over a period of time and may be based on an average of such measurements or an average plus some standard deviation or on some rolling aggregation of all such measurements (e.g. of the (VALUEnew=alpha times VALUEold+beta times MEASUREMENT) where alpha plus beta equals one and MEASUREMENT is the most current SNR measurement). It is not described what happens on the very first occasion that a device, but the teaching seems to be that thereafter a number of measurements will have been taken (including several measurements taken during the showtime of the connection) such that for all subsequent synchronizations a virtual noise template will be selected for use in the synchronization process. The patent teaches away from using quiet line noise measurements taken during a synchronization or training phase since it appears to be believed that this may not capture the noise as well as SNR measurements taken during showtime of a connection. There is no teaching that unless a minimum number of quiet line noise measurements are taken for a line per unit of time, no virtual noise mask is generated for the line. Rather the teaching is that a virtual noise mask is always selected without any dependency upon the number of times that a connection resynchronizes per unit of time.
Furthermore, US 2010/0254442 does not teach a method of selecting which tones to use as break points in the virtual noise mask. As mentioned above, the patent teaches merely selecting from one of a finite number of virtual noise templates which it would seem all use the same predetermined tones for the break points with merely different magnitudes of the break points to generate similarly shaped but differently scaled versions of the common template. Thus there is no suggestion of analyzing the detected noise over the full range of available tones and selecting certain of those tones to use as the break points based on that analysis.